The invention relates to the field of mobile communication systems. More particularly, the present invention relates to a communication system including a central terminal and a plurality of identical mobile surface terminals for communicating command and telemetry data.
There have been several proposed Mars missions that require a networked communication system for communicating unpacketized command and control data between an orbiting spacecraft and a plurality of multiple mobile landers. The communication system can be used to support an investigation of Mars. The networked communication system may include several, for example, ten, small mobile landing vehicles, that is, landers, distributed over the Martian surface to form a global network of communicating mobile landers. The multiple mobile surface terminals are needed for surveying vast surface areas of Mars while communicating command and telemetry data with the orbiting spacecraft. For these Mars missions, more than one lander may be within the beamwidth of a single spacecraft antenna. Each of the mobile surface terminals, or landers, will each need communication transceivers and antennas. The commanding and tracking of the moving landers, as well as the reception of telemetry data is required during surveying operations.
A conventional deepspace networked communications system includes multiple deepspace spacecraft communicating with an earth ground station. Each of the multiple deepspace spacecraft has a respective communication transceiver and antenna for communicating with the single earth ground station. The ground station tracks each of the spacecraft. The communication channel can be characterized as an additive white Gaussian noise channel communicating encoded data using conventional communication encoding, modulation, and channelization techniques. Such systems do not address fading and multiple path problems as the communication is primarily through empty space between the deepspace spacecraft and the ground station. A deepspace networked communications system includes a command uplink and a telemetry downlink. The deepspace networked communications system does not provide for command and control of multiple surface landers using command downlinks and telemetry uplinks.
In deep space applications, the round trip communication time between a deep space spacecraft and a ground station is relatively large as compared to earth orbit communications. The communications uplinks and downlinks use conventional phase modulation signaling. Therefore, multiple spacecraft appear to be stationary within the beamwidth of the ground station antenna. By contradistinction, mobile terminals on the surface may appear to the orbiting spacecraft to be moving within the same beamwidth of the spacecraft antenna as does the personal communications system used on Earth. For deepspace communications, multiple command channels are sent directly from a single ground station. A single ground station receives telemetry data over many communications channels. Therefore, weight and power consumption are not of concern to the communication designers of a ground based system. For a mobile surface lander application, weight and power are of significant design considerations.
In a personal communications system environment, where the users are moving within a single beamwidth of the base station antenna, direct sequence spread spectrum communication methods are used to communicate with several users using communications spreading codes respectively for each personal system. The spreading codes enable code division multiple access communications for separated and isolated reliable communications. The command and telemetry communications are sent from and received by the spacecraft over multiple mobile landers similar in the design to personal communication system handsets. However, the personal communication systems may disadvantageously require ranging signals for determining the distance to the personal mobile terminal, and are not used to communicate command and telemetry data for orbiting spacecraft control of a plurality of mobile surface terminals. Hence, there exists a need for a light weight and low power multiple lander communication system that offers both command and telemetry communication links over separated and isolated communications channels with an ability to communicate with multiple landers in the beamwidth of the central spacecraft system.
In the global positioning system (GPS), clear/acquisition short code (C/A) and pseudo-random nonstandard long codes (PN) are used using BPSK modulators for transmitting code division multiple access signals. The PN code can be acquired as a pilot code for initial code tracking, while the C/A code is used for separating users. The communication channel for multiple mobile lander applications is not an average white Gaussian noise channel because fading and multipath effects are caused by ground terrain upon which are moving the mobile landers. Existing deepspace communication systems can not be directly used for mobile lander applications because fading and multipaths caused by the planet surface may corrupt communications channel signals. The personal communications systems use spread spectrum signals for code division multiple access but disadvantageously lack command and telemetry channels necessary for remote navigation, command and control. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a system for communicating data between a central orbiting system and a plurality of mobile surface terminals.
Another object of the invention is to provide a communication system for communicating command and telemetry data between a spacecraft and a plurality of mobile landers.
Yet another object of the invention is to provide a communication system for communicating command and telemetry data between a spacecraft and a plurality of mobile landers using a command downlink for communications from the spacecraft, and using a telemetry uplink for communications from the landers.
Still another object of the invention is to provide a communication system for communicating data between a spacecraft and a plurality of mobile landers using a command downlink for communications from the spacecraft, and using a telemetry uplink for communications from the landers, using direct sequence spread spectrum communications.
Yet a further object of the invention is to provide a communication system for communicating data between a spacecraft and a plurality of mobile landers using a command downlink and a telemetry uplink using direct sequence spread spectrum communications with a pilot code used for signal acquisition.
The present invention is directed to a code division multiple access networked communication system for communicating command and telemetry data between a central system and a plurality of mobile surface terminals. A terminal is a communications transceiver with antennas in a lander that is capable of moving on the surface of a planet, such as Earth or Mars. The central system is a communications transceiver with antennas in an airborne or spaceborne craft, such as a spacecraft that is typically on-station in a prescribed orbit about the planet on which the landers move. Multiple landers are at times within the beamwidth of a single spacecraft antenna of the spacecraft for enabling simultaneous or concurrent communications with the multiple landers. The communication system includes an uplink and a downlink for two-way communications between the central spacecraft system and the plurality of surface mobile terminals. The uplink is a communications link from the terminals to the spacecraft. The downlink is a communications link from the spacecraft to the terminals. The uplink may be referred to as the telemetry uplink and the downlink may be referred to as the command downlink. The system uses direct sequence spread spectrum spreading for enabling code division multiple access communications for concurrently communicating with multiple landers. The communication system provides for reliable uplink and downlinks communications in the presence of fading and multipath affects due to variable surface terrain and atmospheres through and around which signals are communicated. The communication system includes a downlink subsystem having a spacecraft downlink transmitter communicating a quadrature component carrying a pilot code for coarse acquisition and an inphase component carrying a Walsh-Hadamard code and a PN code for fine acquisition. The downlink subsystem uses spread spectrum signaling and amplitude modulation for code division multiplexing to downlink receivers respectively disposed in a plurality of mobile surface landers. The communication system also includes an uplink subsystem having a spacecraft uplink receiver and lander uplink transmitters communicating code division multiple access telemetry data using spread spectrum signaling and phase modulation. The communication system enables command and telemetry control of a plurality of mobile surface landers.
The communication system has reduced mass, size, and power consumption. The communication system provides increased command and telemetry data throughput with accurate command and telemetry performances using accurate range measurements for the mobile surface terminals. The communication system uses a new set of communications standards for multiple surface lander applications for improved command and control communications. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.